1. Field of the Invention
The present invention relates generally to grids used in a battery and, more particularly, to a stamped battery grid for a lead-acid battery that is configured to optimize electrical performance, reduce weight and maintain battery life characteristics.
2. Description of the Related Art
Grids for lead-acid batteries provide structural support for the active material therein, and also serve as a current collector during discharge and current distributor during recharge of the battery. Accordingly, grid designs seek to optimize the amount of active material supportable by the grid to increase the current collection and distribution characteristics of the grid while minimizing the grid weight. Attempts to optimize the current conducting capabilities while minimizing the weight of the grid have led to numerous grid designs. Manufacturing methods and the disadvantages associated therewith have, however, limited the ability to manufacture even a greater number of grid patterns that have increased conduction capabilities and reduced weight.
Battery grids are commonly manufactured by processes such as casting, expanded metal forming, and stamping. Cast grids have been used for many years and are manufactured by pouring molten lead into a mold, allowing the lead to cool, and then separating the grid from the mold. Cast grids suffer from higher porosity, as compared to expanded metal or wrought grids, and a roughened surface finish. Each of these features may lead to grid corrosion which is a substantial cause of battery failures. Moreover, mold constraints inherent in the casting process limit the wire patterns that may be formed by casting. Further limitations due to mold constraints limit wire shapes and lead distributions that in turn affect grid electrical performance and efficiency. Further disadvantages of the casting process include the need to use a mold coating to facilitate ejection of the grid from the mold, as well as the use of multiple molds to increase production output. These process constraints introduce undesirable grid variations. Finally, the casting process is not "continuous" in the sense that the work material does not pass through the process from start to finish. Rather, work-in-process is collected at each processing station and passed in batches to the next processing stage.
While many disadvantages of the cast grids are addressed by the present invention, of particular concern is the limitations on wire patterns, wire shapes and lead distributions caused by mold constraints. Particularly, casting molds for battery grids generally provide for the infusion of a lead alloy along the horizontal wires of the grid. The lead alloy is introduced into the mold at the ends of recesses for the horizontal wires, and from there the lead alloy flows through the horizontal wire recesses into the connecting vertical wire recesses to form the vertical wires. To ensure complete formation of the vertical grid wires, the spacing between adjacent horizontal wires is limited, thereby limiting the size of the palette that accommodates the paste filler. Moreover, the manufacturing limitations of casting requires that the horizontal wires be continuous and parallel to one another thereby further limiting the grid patterns manufacturable by this process.
Grids are also formed by expanding metal through a process in which a strip of cast or wrought lead material is pierced and then pulled or expanded. Expanded metal grids offer higher productivity than cast processing because the expanded metal process is continuous, i.e., a strip of lead material enters the process and finished grids are the output thereof. However, expanded metal grids are also limited in wire pattern, wire shape, and lead distribution. Additionally, expanded metal grids include stress zones created from the expansion which lead to corrosion. While corrosion may be reduced through the addition of precious metal additives, such as silver, the off-set in corrosion results in an increase in manufacturing costs.
U.S. Pat. No. 5,582,936 issued to Mrotek, et al., assigned to the assignee of the instant invention and herein incorporated by reference, discloses a grid for a lead-acid battery plate that has been formed by a casting process. The Mrotek et al. battery grid includes features to optimize the current flow in the grid, while reducing the amount of lead in the grid to keep the grid weight at a minimum. However, the Mrotek et al. battery grid in the '936 patent suffers from the various disadvantages discussed above that are inherent in the grid casting process.
The present invention incorporates some of the techniques in the '936 patent to optimize the electrical performance and reduce the weight of the grid, and includes additional features to provide other characteristics that are not possible in a cast type grid.